The Computer: A Digital Tool

The computer A computer is a computationally complete system - capable of solving all of the problems that can be solved algorithmically. To be practically applicable, however, it not only has to be complete, but to be fast and reliable as well. The electronic computer serves this purpose by means of digital technology.  

Digital Technology Based upon bi-stable electronic components Always in 1 of 2 states: conventional to represent these states using digits 0 and 1 Relationship between electronic components and the binary number system.

Binary system (1) Uses only two digits: 0 and 1, to represent numbers. Larger numbers are represented as sequences of digits, in which each position in the sequence is associated with a different weight.

Value of digit sequence: 128+0+32+0+0+0+2+1 = 163 Binary system (2) This sequence of digits 10100011 represents the number 163: Digit sequence: 10100011 Value of digit sequence: 128+0+32+0+0+0+2+1 = 163

Binary system (3) A single Binary digIT is called a bit. In practice, the circuits of contemporary computers are commonly based upon sequences of 8 bits, called bytes. The largest value that can be represented by a single byte is 255 (digit sequence: 1 1 1 1 1 1 1 1):

Binary system (4) Try working out the sequences that give you a value of : (i) 162  (ii) 97

DIGITAL STORAGE: THE COMPACT DISC (CD)

CDs Most cost-effective method available today of storing and distributing multimedia applications Introduced in 1981 to store music – could play high-fidelity music over and over Also able to hold digital data – became computer storage device First CD-ROMS slow and expensive – data transfer from CD to computer was 1X (150 KB/sec); now 12X available (1800 KB/sec)

CD: Format An optical device – read by a laser beam (vs. magnetic device like hard disk/floppy disk or diskette) Some types of compact disc that hold different data formats: CD-DA (CD-Digital Audio) CD-ROM CD-R (CD-Recordable) VideoCD CD-RW

CD-ROM (CD-Read Only Memory) Most widely used format Holds digital data that can be read by different computers with a CD-ROM drive, with appropriate platform Hybrid CDs – playable on multiple platforms

CD: Physical attributes Diameter of 12 cm; 1.2 mm thick; weighs 14 – 33 grams; hole at centre 15 mm in size   Made of plastic, with layer of aluminium One side for data, one side for label

Data storage on a disc A CD-ROM stores in pits and lands:   A pit disperses light A land reflects light  Light reflectability forms the binary digits 1 and 0 that a computer needs to process data  The more data, the more pits and lands CD Surface Pit Pit Land Land

Advantages of CD-ROM Availability of large installed-base – easy distribution   Large storage capacity – average CD-ROM holds 650 MB of data (> 470 1.44 MB floppy disks) Able to play audio and video Read-only medium: data can be read, not written to ð Immune to viruses ð Cannot be erased ð  Durable – lifespan of 100 years ð Cannot ‘crash’ like hard drive – surface does not wear  

Disadvantages of CD-ROM Can only read data – cannot be updated or changed   Duplication: copies will contain same (unerasable) problems ‘Coasters’ – editing and testing important Slow access time (compared to hard drive)